An intense coherent light such as a laser beam can be passed through a substance having a secondary nonlinear optical effect to generate a light wave with twice the frequency of the input light, or a light wave with a frequency that is the sum or difference of the frequencies of two input components if the input light includes a plurality of frequency components. These techniques are individually called second harmonic generation, sum frequency generation, and difference frequency generation. Although the direction of the generated light wave may sometimes differ from that of the input laser light, that is, the excitation light (or fundamental light wave), it is normally made to be the same as the direction of the excitation light in order to enhance the conversion efficiency by increasing the power of the generated light wave. It is therefore necessary to reseparate the generated light wave from the excitation light (fundamental wave) in order to use only the frequency of the generated light wave. For this purpose, as shown in FIG. 1, for example, proposals have been made to use a dichroic mirror 1 that transmits one frequency and reflects the other frequency (see FIG. 1 ), or a wavelength-selective element such as a prism 2 or a diffraction grating 3. Alternatively, a polarizing beam splitter 4 or the like has been used, taking advantage of the fact that the generated light wave differs in polarization from the fundamental wave. In FIG. 1, the numeral 5 designates a laser generation source, and numeral 6 denotes a wavelength conversion apparatus. To obtain maximum output power, it is necessary to reduce the power losses of the fundamental wave A and the generated light wave B to a minimum in these wavelength-selective elements 1 to 4.
Although a dichroic mirror 1 can generally reduce the power losses of both the fundamental wave A and the generated light wave B, it is sometimes difficult to fabricate a mirror that reduces power losses depending on the wavelength region. In order to separate two wavelengths, it is necessary that the mirror should have a transmittance as close to 100% as possible at one wavelength and a reflectivity as close to 100% as possible at the other wavelength. However, for example, for a blue 500 nm fundamental wave A and a generated light wave B with double that frequency (250 nm), it is rather difficult in practice to increase the reflectivity at 500 nm and at the same time to obtain a transmittance of 80% or more at 250 nm. In particular, although such a mirror is necessary for an intracavity frequency converter, only 80% or less of the generated power will be effectively obtained, because of the limitation of the mirror.
A prism 2 can substantially reduce the power loss of both the fundamental wave and the generated light wave. However, if such an optical system is to be incorporated in an apparatus for high-efficiency intracavity frequency conversion, the number of optical devices in the resonator will be increased by the number of prisms 2. One problem is that even a minute increase in the power loss due to the increase in the number of devices has a substantial impact on the prisms effect of increasing the power generated by the resonator.
Although separation is easier when a diffraction grating 3 is used, because the latter provides high-wavelength dispersion, it is of almost no use in intracavity frequency conversion in which even the slightest loss is a problem, since its reflectivity is not sufficiently high.
It would also be very difficult to fabricate a polarizing beam splitter 4 with a reduced loss; even if such a splitter was realized, it would be very expensive.
Therefore, in most prior art intracavity frequency conversion, dichroic mirrors have been used in place of other mirrors, to allow the resonator to eliminate the generated light wave. However, the specifications required for such dichroic mirrors are rather strict; the reflectivity of the fundamental wave must be as high as possible, the transmittance of the generated light wave must be as high as possible, and the mirrors must be able to sufficiently withstand the high power in the resonator.
On the other hand, the inventors have proposed an intracavity frequency conversion method in which the cross-section of the beam focus in the wavelength conversion crystal is elliptical (Japanese Patent Application No. 3-159530/1991). This invention is also disclosed by Yoichi Taira in "High-Power Continuous-Wave Ultraviolet Generation by Frequency Doubling of an Argon. Laser", Jpn. J. Appl. Phys., Vol. 31, 1992, pp. L682-L684.
In the proposed method, it is very important to design an optical system that causes a minimum loss of the fundamental wave and that can produce a light wave of a newly-generated wavelength most efficiently. To achieve this, an optical system using dichroic mirrors is normally used. However, when the generated light wave is in the ultraviolet region, where the proposed method can be utilized most effectively, it is not always easy to obtain an appropriate mirror. Currently, a mirror for transmitting ultraviolet or totally reflecting green light suffers a heavy power loss due to scattering because of the increased number of layers in the dielectric coating, and the ultraviolet transmittance is about 70% at most. Furthermore, the generated ultraviolet light may degrade optical materials when applied to them. A simple way of avoiding this by separating ultraviolet light from its fundamental wave is thus required.
Therefore, an object of the present invention is to provide a wavelength conversion apparatus that can separate an optical beam generated by a nonlinear optical element from a fundamental beam by using a simple mechanism.
Another object of the present invention is to reduce the power loss associated with the wavelength separation and improve the performance of the wavelength conversion apparatus.
The present invention can be applied not only to a wavelength conversion apparatus using a resonator, but also to a wavelength conversion apparatus that has no resonator.